The invention is directed to a method for the operation of an electrophotographic printer or copier device. The invention is also directed to a printer or, respectively, copier for the implementation of the method. This printer is explained later. Methods are known wherein a charged photoconductor region is illuminated with a predetermined illumination energy by an illumination device. The electrical potential on the photoconductor region is acquired after the illumination. Taking the acquired potential into consideration and on the basis of an approximation for the current characteristic of the photoconductor, an adapted illumination energy is determined that opposes the influence of deviations of the current characteristic of the photoconductor from a rated characteristic on the printing event.
For example, the characteristic of the photoconductor deviates from the rated characteristic due to temperature changes, wear or aging of the photoconductor. Given replacement of the photoconductor, there are also deviations in the characteristics to be attributed to production tolerances.
A method for determining the illumination energy is explained in DE 196 12 637. The known type of illumination control only allows a balancing or, respectively, a weakening of the effects of the deviating characteristic in view of solid white or, respectively, black surfaces. The quality of finer image details is not designationally influenced.
DE 195 09 852 C2 discloses a method for the control of the image quality wherein the quality of fine image details is acquired in predetermined standard patterns with the assistance of an optical sensor or of a potential sensor and is subsequently controlled by modifying parameters of the printing event. The sensor must thereby acquire the print quality of individual image features. The type of control of the parameters is only specified in general.
An object of the invention is to specify a method for the operation of an electrophotographic printer device wherein large surfaces and fine details in the print image as well can be printed with high print quality even given an altered characteristic of the photoconductor. A further object of the invention is to specify a printer or, respectively, copier device that prints large surfaces and fine details with high print quality even given an altered characteristic.
This object is achieved by a method having the method steps recited in patent claim 1. Developments of this method are recited in the subclaims dependent on claim 11.
The inventive method proceeds from the perception that variations in the discharge behavior of the photoconductor lead to variations of the electrical field above the photoconductor. The field above the photoconductor attracts the toner particles and therefore influence the image development. An approximately constant field strength that is only defined by the difference in potential between the photoconductor surface and the developer station prevails on photoconductor surfaces that correspond to larger white or, respectively, black image areas. In addition to being dependent on said difference in potential, the electrical field on photoconductor surfaces that correspond to fine image features, for example thin lines, discrete and raster dots, is also dependent in a complicated way on the geometrical shape of the details and on the physical properties of the photoconductor. This means that variations of the photoconductor properties influence the inking of large areas and fine image details in a different way.
In the inventive method, only a first part of the image features of a charged photoconductor region is illuminated with a predetermined illumination energy. The other part of the image features of the photoconductor region employed for the determination of the adapted illumination energy is not illuminated or, respectively, is illuminated with a significantly lower illumination energy. The ratio of the plurality of the image features of the first part to the plurality of image features of the second part defines a raster parameter. This raster parameter is employed in the determination of the adapted illumination energy. The high print quality of large areas and fine image details given different photoconductor characteristics in the inventive method is achieved in that the potential profile of an image detail is matched to a target form by modifying the illumination. The acquisition of the potential of individual picture elements, however, is avoided because a region composed of a plurality of image features, namely of the image features of both parts, is employed in the inventive method for measuring the potential with the assistance of a sensor that integrates over the area. The measurement of potential ensues simply and exactly given an adequately large, selected region of, for example, 25 mm2.
In a development of the inventive method, the image features of the first part and the image features of the other, second part as well are approximately uniformly distributed on the photoconductor region, for example like a checkerboard or stripe-shaped. As a result of this arrangement of the image features, an average field strength that is very well-suited for the determination of the adapted illumination energy is acquired in the measurement of the potential. A uniform distribution of potential arises on the photoconductor region, so that slight positional deviations of the potential sensor have no influence on the measured result.
In a next development, a multi-level character generator as disclosed, for example, in U.S. Pat. No. 5,767,888 is employed for the illumination. The multi-level character generator is realized either with the assistance of an LED line, as in said Letters Patent, or by employing a multi-level laser that scans the photoconductor.
The invention is also directed to a printer for the implementation of the inventive method. The aforementioned technical effects therefore also apply to the inventive printer.